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    <title>Document</title>
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<body>
    <script type="text/javascript">
          const users = {
            Alex: {
                email: 'alex@alex.com',
                skills: ['HTML', 'CSS', 'JavaScript'],
                age: 20,
                isLoggedIn: false,  //是否
                points: 30
            },
            Asab: {
                email: 'asab@asab.com',
                skills: ['HTML', 'CSS', 'JavaScript', 'Redux', 'MongoDB', 'Express', 'React', 'Node'],
                age: 25,
                isLoggedIn: false,
                points: 50
            },
            Brook: {
                email: 'daniel@daniel.com',
                skills: ['HTML', 'CSS', 'JavaScript', 'React', 'Redux'],
                age: 30,
                isLoggedIn: true,
                points: 50
            },
            Daniel: {
                email: 'daniel@alex.com',
                skills: ['HTML', 'CSS', 'JavaScript', 'Python'],
                age: 20,
                isLoggedIn: false,
                points: 40
            },
            John: {
                email: 'john@john.com',
                skills: ['HTML', 'CSS', 'JavaScript', 'React', 'Redux', 'Node.js'],
                age: 20,
                isLoggedIn: true,
                points: 50
            },
            Thomas: {
                email: 'thomas@thomas.com',
                skills: ['HTML', 'CSS', 'JavaScript', 'React'],
                age: 20,
                isLoggedIn: false,
                points: 40
            },
            Paul: {
                email: 'paul@paul.com',
                skills: ['HTML', 'CSS', 'JavaScript', 'MongoDB', 'Express', 'React', 'Node'],
                age: 20,
                isLoggedIn: false,
                points: 40
            }
        }
    
        // console.log(nameArr);
        // 1.统计在线人数
    const nameArr = Object.keys(users)  //keys:返回的是键的数组
        count = 0
        nameArr.forEach(key => users[key].isLoggedIn ? count++ : count)   //遍历
        console.log('在线人数' + count);




        // console.log(arr);
        // 2.统计分数大于50的人数

        // 3.查找会MongoDB的人，并打印其名字
        const arr = []
        nameArr.forEach(key => users[key].skills.includes('MongoDB') ? arr.push(key) : arr)
        console.log('会MongoDB的人' + arr);
        // 4.查找同时会：MongoDB， Express，React，和Node 的人，并打印其名字
        const arr1 = []
        nameArr.forEach(key => users[key].skills.includes('React', 'JavaScript') ? arr1.push(key) : arr1)
        console.log('会MongoDB的人' + arr1);




        var collection = {
    "1275":{
        "album":"依然范特西",
        "artist":"周杰伦",
        "tracks":[
            "听妈妈的话",
            "千里之外"
        ]
},
    "2468":{
        "album":"西界",
        "artist":"林俊杰",
        "tracks":[
            "西界",
            "杀手"
        ]
},
    "3245":{
        "artist":"许嵩",
        "tracks":[ ]
    },
    "5439":{
        "album":"遇见"
    }
}

// 1.定义方法更新记录:
// function updateRecords(id,prop,value)
// 要求：
// 1).如果传入的value是一个空值，表示将该属性删除
// 2).传入的prop如果是tracks，则代表添加，其他是修改
  function updateRecords(id,prop,value){
        value=='' ? delete collection[id][prop] :(prop=='tracks' ?collection[id][prop].push(value) :collection[id][prop]=value )  ;
    }
    updateRecords(3245,'tracks','有何不可')
    console.log(collection);
    updateRecords(3245,'tracks','')
    console.log(collection);
    updateRecords(5439,'album','逆光')
    console.log(collection);

// 3245 tracks '有何不可'

// 3245 tracks ''   -->删

// 5439 artist '孙燕姿'

// 5439 album '逆光'


    </script>
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